Structural and Optical Properties of Sol-Gel Deposited Proton Conducting Ta₂O₅ Films

Proton conducting tantalum oxide films were deposited by spin coating using a sol-gel process. The coating solutions were prepared using Ta(OC₂H₅)₅ as a precursor. X-ray diffraction studies determined that the sol-gel films, heat treated at temperatures below 400 °C, were amorphous. Films heat treated at higher temperatures were crystalline Ta₂O₅. The solar transmission values (Ts) of tantala films on glass generally range from 0.8-0.9 depending on thickness. The refractive index and the extinction coefficient were evaluated from transmittance characteristics in the UV-VIS-NIR regions. The refractive index values calculated at λ=550 nm increased from n=1.78 to 1.97 with increasing heat treatment from 150 to 450 °C. The films heat treated at different temperatures showed low absorption with extinction coefficients of less than k=1 x 10^-3 in the visible range. Spectrophotometric and impedance spectroscopic investigations performed on Ta₂O₅ films revealed that these films have protonic conductivity of 3.2 x 10^-6 S/cm. The films are suitable for proton conducting layers in electrochromic (EC) devices.